Bleaching is the process of removal of any undesired

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Bleaching is the process of removal of any undesired colouring from fabric before dyeing or printing. Bleaching uses three major chemical procedures, namely sodium chlorite, sodium hypochlorite or hydrogen peroxide bleaching, with the fi rst two procedures being the most common ones. These bleaching agents are carried in with water usually to either dilute or for better reaction with fabric due to the ease of adsorption on fabric. Chlorine dioxide acts as the oxidizing agent. However, chlorine dioxide being a powerful oxidizing agent is corrosive and highly harmful in nature. Therefore, proper disposal and treatment of this ef fl uent are highly necessary. 3 Major Pollutants The different processes in the textile industry lead to discharge of various toxic pollutants into the water stream which can be harmful. It is important to understand the nature of these pollutants in order to ensure the proper removal and application of the right treatment method (T ü fekci et al. 2007 ). The major pollutants are the organic chemicals such as azo dyes, pulp, gum, cellulose, hemicellulose and alkali. Water from dyeing and printing process accounts for more than 50 % of the total ef fl uent (Saxena and Kaushik 2011 ). Pre-treatment of polyester fi bres is done by reduction reaction between the polyester fi bre and 8 % sodium hydroxide at 90° for duration of 45 min. This leads to decomposition of the fi bre into terephthalic acid and ethylene glycol. The COD of the resulting ef fl uent accounts for about 60 % of the ef fl uent from dyeing and printing processes even though it accounts for only 5 % by volume of the total ef fl uent (Razzak 2014 ). Chromium is another kind of pollutant in wastewater which causes a lot of concerns. Chromium is usually used as a catalyst or as chromium dyes for wool industries or comes from potassium dichromate used for tanning. Depending on the type of dye and process being employed, the chromium content is 200 500 times more in ef fl uent than earlier due to the dyeing rate being ceased after process (Wang et al. 2011 ). 64 P. Senthil Kumar et al.
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pH is another factor which needs to be considered in wastewater from dyeing processes. During the processes of scouring, desizing and mercerization, which are carried out before printing and dyeing, the pH of ef fl uent wastewater stays around 10 11 when treated with alkali at 90°. As already mentioned, reduction of polyester fi bres uses sodium hydroxide base which has a pH ranging between 10 and 11 (Menezes and Choudhari 2011 ). Therefore, the ef fl uent wastewater is usually alkaline in nature and the fi rst treatment process includes adjusting the pH value of the dyeing wastewater. Nitrogen in ammonia and urea is also major pollutants and harmful. Batik and other complicated techniques use urea. The total nitrogen content in urea is 300 mg/L, which can be dif fi cult to treat. Phosphorus is also present in wastewater which usually comes from the phosphor-containing detergents and monosodium phosphate used as buffer. Phosphorous can lead to rapid eutrophication of water bodies and therefore must be kept under check. Sulphide mainly comes from the
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  • Fall '19
  • N. Gokarneshan

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